<!DOCTYPE html>
<style>
#outer {
    position: relative;
}
#inner {
    top: 100px;
    left: 100px;
    width: 100px;
    height: 100px;
    opacity: 0.5;
    position: relative;
    background-color: green;
}
</style>
<div id="outer">
  <div id="inner"></div>
</div>